The easiest way I've found by far to install Ruby on CentOS is by using the Software Collections - here's a link for the Ruby 2.2 collection.

This is how I did it on CentOS 6.x:

1. First install the collections utilities:

yum install scl-utils -y

2. Then install the Ruby 2.2 collection repository:

yum install https://www.softwarecollections.org/en/scls/rhscl/rh-ruby22/epel-6-x86_64/download/rhscl-rh-ruby22-epel-6-x86_64.noarch.rpm -y

3. Now use that repository to install Ruby:

yum install rh-ruby22 rh-ruby22-ruby-devel zlib-devel -y

4. Also install the CentOS Development tools group so you don't hit any dependencies issues:

yum groupinstall Development tools -y

5. Add the correct paths in /opt/rh/rh-ruby22/enable:

echo 'export PATH=/opt/rh/rh-ruby22/root/usr/local/bin${PATH:+:${PATH}}' >> /opt/rh/rh-ruby22/enable

6. Now add the enable script on your ~/.bashrc so that Ruby is automatically enabled every time you login:

echo "source /opt/rh/rh-ruby22/enable" >> ~/.bashrc

7. Finally source the enable script so you can start using Ruby 2.2 right away:

source /opt/rh/rh-ruby22/enable

8. Test:

ruby -v
ruby 2.2.2p95 (2015-04-13 revision 50295) [x86_64-linux-gnu]
gem -v
2.4.5

Awesome :)